Crosswell seismic tomography is a recently developed technology which permits the structure of the subsurface formations between two spaced apart wells to be accurately determined. Generally, a downhole seismic source is placed in a first well and a plurality of downhole seismic receivers are placed in a second well. The seismic source in the first well is then activated to generate a seismic signal which propagates through the subsurface formations to the seismic receivers in the second well where it is recorded. Typically, the seismic source is then moved vertically downwardly or upwardly in the first well and reactivated, and the resulting seismic signal is again recorded by the seismic receivers in the second well. This procedure is repeated until seismic data covering the entire interwell region of interest has been generated. The resulting seismic data may be processed to yield information on the subsurface formations through which the seismic signals passed. U.S. Pat. No. 4,214,226 issued Jul. 22, 1980 to Narasimhan et al. provides a general description of crosswell seismic tomography.
Crosswell seismic tomography has a number of potential uses in the oil and gas industry. For example, crosswell seismic data may be used to determine the interwell seismic velocity and absorption fields. This information may then be used to determine reservoir characteristics, estimate reservoir properties, and monitor the effectiveness of enhanced oil recovery processes. Other potential uses of crosswell seismic data will be well known to those skilled in the art.
A number of downhole seismic sources have been developed to enable the acquisition of crosswell seismic data, including downhole vibrators, resonators, explosive sources, piezoelectric transducers, magnetostrictive transducers, implosive sources, downhole airguns, and sparkers. All of these downhole seismic sources, however, must be designed to ensure that they do not damage the wellbore, which places strict upper limits on their power output and, accordingly, on the strength of the resulting seismic signals. This limitation restricts utilization of crosswell seismic tomography to situations in which the interwell distance is relatively short (i.e., no more than about 1,000 feet).
Another limitation on the use of crosswell seismic tomography is the substantial costs associated with preparing the wells for deployment of downhole seismic sources and receivers. In most cases, the diameters of the downhole seismic sources and receivers are too large to fit into the production tubing used to convey fluids from the reservoir to the surface. Therefore, at most sites, the production tubing must first be removed from the wells in order to conduct a crosswell survey and then be reinstalled following completion of the survey. This is a time-consuming and expensive process. It should be noted that recently a new type of intra-tubing hydrophone has been developed which may eliminate the need to remove the production tubing from the well in which the downhole seismic receivers are located. One example of such an intra-tubing hydrophone is the slim hole hydrophone array developed by Innovative Technologies Inc. of Houston, Tex. Nevertheless, in order to conduct a conventional crosswell survey it will still be necessary to remove the production tubing from the well in which the downhole seismic source is located.
From the foregoing, it can be seen that there is a need for a method of acquiring crosswell seismic data in which the strength of the seismic signal can be increased without fear of damaging the wellbore. There is also a need for a method of acquiring crosswell seismic data which avoids the necessity of removing the production tubing from the wellbores. The present invention satisfies these needs.